The present invention relates to improvements in fastener strips of the type formed of an extruded plastic with an interlocking profile thereon shaped to interlock with another profile.
Fastener strips of this kind are normally utilized by attaching to a film of plastic such as of sheet material or where the plastic is formed into a container. The fastener strip is frequently bonded to the plastic film by heat to cause the plastic of the strip and of the film to weld to each other.
It has been found, particularly with heavy duty fastener applications, that the fusion temperatures required to bond the fastener onto a film will cause the fastener web and/or the fastener itself to elongate beyond normal tolerances. In the attachment of the fastener strip to the film in order to obtain intimate contact between the film and the strip, longitudinal forces are applied both to the film and the strip to remove wrinkles and to insure intimate surface contact. In the application of the longitudinal forces concurrent with the application of heat to cause bonding, the fastener may elongate beyond normal tolerances as it enters a state of flux or yieldability at the time of bonding to the sheet material, resulting in a distortion of the profile and/or a curvature or warping of the line of contact between the strip and the film.
In many practical applications, it is impossible to control the longitudinal tension applied to the strip required for obtaining the intimate contact between the strip and the film and even allowing for sophisticated tension systems, the rate of elongation of the material with the application of heat cannot be adequately controlled. This is particularly true in the case of materials of different thicknesses which have been manufactured under different conditions and at different draw-down rates. In the attachment of a continuous fastener strip to a film with the application of heat, a longitudinal force or stretching force may be applied to the strip and applied simultaneously to the film. Heat is applied at the area where the web of the strip overlies the film to cause a fusion of the plastic and a bonding of the strip to the film. The longitudinal force is practically applied to the web of the strip and due to the softening of the plastic on the web of the strip, the web will elongate whereas the cooler profile at its side will not elongate thereby causing a curvature in the strip as an entity. The elongation of the web can also be transferred to the fastener profile at the edge causing additional distortion. This distortion is undesirable in the final product and further problems and distortion of the two structures will occur upon cooling where increasing distortion and uneven matching of the closure to its opposing joining member result.
It is accordingly an object of the present invention to provide an improved fastener strip and method and apparatus for making the strip wherein a non-extensible reinforcement is associated with the fastener strip and especially the web of said strip so that a longitudinal force can be applied to it while being attached to a film without distortion and wrinkling of the final product.
A further object of the invention is to provide an improved method and apparatus for making a fastener strip and attaching the fastener strip to a film wherein a more uniform, stronger and satisfactory attachment results and the finished product lies smooth without distortion of the fastener profile at the edge of the strip.
A further object of the invention is to provide an improved structure and method of making a fastener strip with a non-extensible reinforcement in the web of the fastener strip wherein the reinforcement can be associated with the strip or attached thereto in relatively high speed production procedures and without adversely affecting the speed or quality of production.